#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <string>
#include <sstream>
#include <complex>
#include <algorithm>
#include <functional>
#include <vector>
#include <stack>
#include <queue>
#include <set>
#include <map>
#include <cmath>
#include <typeinfo>
#include <cassert>
#include <fstream>
#define REP(i,n) for(int i=0;i<n;i++)
#define FOR(i,a,b) for(int i = a;i<b;i++)
#define SSTR( x ) static_cast< std::ostringstream & >( \
        ( std::ostringstream() << std::dec << x ) ).str()
#define ALL(s) (s).begin(), (s).end()
using namespace std;
typedef long long unsigned int llu;
typedef long long ll;

int main (){
  int n;
  vector<long int> x;
  while (cin >> n){
    bool b = false;
    REP(i,n){
      long int a;
      cin >> a;
      if(find(ALL(x),a) != x.end()){
	b = true;
      }
      x.push_back(a);
    }
    if(b){
      cout << "NO" << endl;
      continue;
    }
    sort(ALL(x));
    long int d = x[1] - x[0];
    FOR(i,2,n){
      if(x[i] - x[i-1] != d ){
	cout << "NO" << endl;
	b = true;
	break;
      }
    }

    if(!b) cout << "YES" << endl;
  }
  return 0;
}